Northwest of Honolulu, Lake Wilson has great opportunties for fly fishing. Lake Wilson (aka Wahiawa Reservoir) was created in the early 1900s when the North and South Forks of Kaukonahua Stream were dammed as a sugar cane irrigation source. It is the only place for freshwater fishing on Oahu.
Lake Wilson has been stocked with peacock and largemouth bass. Plus, due to dumping, you will find plentiful tropical aquarium type fish.
Since there is limited public access, shore fishing is very difficult. Boating is the way to go.
If you are going after bass, then you will want to try fishing in September and October.
Note: A valid Freshwater Game Fishing License is required.